Electromagnetic radiation at Terahertz (THz) frequencies and sub-millimeter wavelengths are being investigated for various applications, such as, low-loss interconnects, sub-wavelength imaging, and high sensitivity spectroscopy and biosensing. One of the challenges for the success of THz technologies is transporting the waves at THz frequencies over sufficiently long distances, e.g., many orders of magnitude greater than the sub-millimeter wavelengths. Various waveguides have been examined to transport the waves with little promise due to substantial wave attenuation and dispersion that causes signal loss and distortion. The observed wave attenuation for such waveguides is not suitable for sensing and communications systems, for instance, in comparison to optical fiber waveguides that are typically used in telecommunications to transport optical waves with shorter wavelengths.
One examined waveguide that has relatively lower attenuation at THz frequencies is the parallel plate waveguide (PPWG), which comprises two parallel plates. The PPWG has no low frequency cutoff for THz waves corresponding to a transverse electric and magnetic (TEM) mode and therefore can transport the TEM mode with little or no wave dispersion. However, the PPWG attenuation for the TEM mode may increase as the wave frequencies increase, which causes signal losses and makes it unsuitable for broadband applications. For modes other than the TEM mode, the PPWG may have THz cutoff frequencies that cause significant wave dispersion and hence pulse distortion. The distortion for a mode can be reduced if its cutoff frequency is decreased, for instance, by further separating the two parallel plates of the PPWG from one another. However, increasing the distance between the parallel plates also allows additional modes to propagate, which have higher cutoffs and hence introduce additional dispersion.